Field of the Invention
This invention relates generally to frequency-to-binary converters, and more particularly to a converter adapted to convert accurately and at high speed an input signal whose frequency varies as a function of a parameter being measured into a bit parallel number for manipulation in a digital computer.
In a typical process control loop, a fluid is conducted through a control valve or final control element into a process load. An automatic electronic controller is a component in a process control loop which is subject to disturbances, the controller acting to maintain a process variable, such as flow rate, temperature, pressure, humidity or liquid level, at a desired value. To accomplish this purpose, the controller receives both the desired value or set point and the process variable and compares these values to produce an output signal that depends on the error therebetween, which output signal serves to govern the final control element to correct this error. Automatic controllers are generally classified by the type of control action or modes of control they perform, such as the proportional position mode, proportional plus reset and proportional plus rate.
In place of individual process control loops, one for each parameter being sensed and controlled, it is now the practice in many large chemical processing and other industrial plants to make use of a direct digital control system (DDC) which employs a digital computer on a time-sharing or multiplexing basis to carry out the functions that previously had been performed by individual electronic controllers, one for each loop.
Since in the typical process control loop the parameter being measured by the primary transducer is represented by an analog signal and the final control element is operated by an analog signal obtained from the electronic controller, in a DDC system associated with a group of primary transducers and a related group of final control elements, it is necessary to convert the analog signals from the primary transducers into corresponding digital values for manipulation by the digital computer and to convert the digital output of the computer into analog signals for operating the final control elements.
When, for example, the process variable is a changing temperature which the primary transducer represents as a voltage or current signal of corresponding magnitude or intensity, this input signal must be converted into a digital value in a DDC system. In this way, the digital computer in the system, which is adapted to execute a program carried in its memory, is then able to manipulate the digital value to produce an output for carrying out the desired control function. When, however, the process variables being sensed by the primary transducers are represented by signals whose frequencies vary as a function of the parameters being measured, then the DDC system must include a frequency-to-binary converter.
Thus a vortex-shedding or Swirlmeter type of flowmeter for measuring the rate of fluid flow through a flow tube produces an output signal whose frequency depends on flow rate. Such flowmeters are disclosed in the Herzl U.S. Pat. No. 3,854,334 and in the patents referred to therein. Similarly, turbine meters yield an output signal that is proportional to flow rate.
When the input signals applied to a DDC system are derived from a group of primary transducers yielding signals of varying frequency, these signals must be applied to a multiplexer which acts to scan the signals in sequence and to feed each signal sample into a frequency-to-binary converter which converts each sample into a bit parallel binary number that is read by the digital computer.
The digital output of the computer in the DDC system in response to each incoming signal sample is converted into an analog signal sample for operating the final control element in the process control loop which includes the primary transducer producing the incoming signal in question. In the output of the system, the analog signal samples which appear in sequence in the course of a multiplexer scan cycle must be held at their existing values until the next scan cycle, so that the final control elements are maintained in accordance with the existing values until new values are yielded by the computer. This holding action also applies to the digital readouts of the computer.
The ability of a DDC system to handle a very large number of process control loops on a multiplexing basis depends mainly on the speed and accuracy of the frequency-to-binary converter. If this conversion can be carried out accurately at high speed, then many incoming frequencies can be scanned and processed in the course of a brief scanning cycle. Since the respective analog and digital outputs of the computer are maintained constant between scanning cycles, this effectively results, for all intents and purposes, in simultaneous real time outputs.
To give a rough example of the relationship between the time it takes to effect frequency-to-binary conversion and the ability of a DDC system to handle a large number of input frequencies, let us assume that there are 100 input frequencies to be scanned in each multiplexing cycle, and each conversion takes one tenth of a second. In this case, it would take 10 seconds to complete a scan, and there would be, with respect to each analog output sample, a full 10 second pause between successive samples. This would certainly not begin to approach a real time output. On the other hand, if it only took a thousandth of a second to complete a frequency-to-binary conversion, then there would only be a 1/10th second pause between successive samples, and this would approach real time conditions.
In order, therefore, to render practical a DDC system for handling input signals from the primary transducers which are of the variable frequency type, it is essential that the frequency-to-binary conversion be fast and accurate.
Two widely used methods are known to accomplish frequency-to-binary conversion. The first known technique is based on the frequency counter approach and simply acts to count the number of cycles that occur in a predetermined time period, the count thereby providing a number representing frequency. The main drawback to this technique is that to obtain high resolution, a large number of cycles must be counted. Thus to resolve one part in 1,000, one must count 1,000 cycles. If the signal frequencies are in a high frequency range, the drawback is not serious; but for low frequencies, the counting time period is necessarily quite long.
The second known technique is much faster, for it effects frequency-to-binary conversion by measuring the time required for completing a single cycle of the frequency being converted. But this presents a serious problem when high frequencies are involved, for then very small increments of time must be resolved. For example, if the incoming frequency is 1000 Hz, the period to be measured is 0.001 seconds. To measure 0.001 seconds with a resolution of 0.1% requires resolving one microsecond.
Both known conversion methods suffer from the disadvantage that the time to complete the measurement varies with the incoming frequency, so that high frequencies can be measured quickly, whereas lower frequencies take proportionately longer times. This can be a serious drawback when the time available to carry out the measurement is limited and cycle-to-cycle jitter of the incoming frequency dictates period averaging.
The following prior patents are of interest in connection with frequency-to-binary conversion: U.S. Pat. No. 3,928,797 to Kiencke; U.S. Pat. No. 3,929,798 to Valis; U.S. Pat. No. 3,829,785 to Schroder et al.; U.S. Pat. No. 3,997,764 to Belmonte et al. and U.S. Pat. No. 3,609,756 to Hallsall. The Schroder patent is of particular interest, for it discloses a converter in which the frequency and period of a signal is measured during a measuring time interval with the aid of a chain of counting circuits and an arithmetic unit connected to the outputs thereof.